1. Field of the Invention
The present invention relates to a tension member for structures, and also to a method for the manufacture thereof.
2. Description of the Background Art
Generic tension members are known in civil engineering, especially in connection with cable-stayed bridges and suspension bridges. But such tension members are also used for concentrated load transmission in producing roof constructions, for example when covering stadiums with roofs.
In general, generic tension members include a plurality of tension elements, for example steel rods, steel wires, or stranded steel wires, which run inside a tubular sheath. In order to protect against corrosion, the individual tension elements are provided with a suitable coating, and may additionally be arranged in a plastic casing. A bundle of such tension elements is additionally surrounded by a tubular sheath, generally of polyethylene, firstly in order to protect the tension elements from mechanical influences, and secondly to further improve corrosion resistance.
During manufacture of such tension members, the individual tension elements are generally tensioned gradually, one at a time, within the tubular sheath between the two anchor points connected by the tension member. A certain remaining unoccupied area is left between the tension elements and the inner wall of the tubular sheath for installation of the individual tension elements within the tubular sheath. This remaining unoccupied area also permits later replacement of tension elements during maintenance and repair or later augmentation of a tension member with additional tension elements to increase the load capacity of the structure.
However, one consequence of this type of construction is that under certain circumstances, such as when a wind load is present, the tubular sheath and the tension elements extending within it move relative to one another in the transverse direction, which can cause banging and clattering noises, but which also signifies an additional dynamic stress on the tension member.
From WO 2005/049923 A1, which corresponds to U.S. Publication No. 2007061982, a device is known for damping the vibrations of the tension members of a cable-stayed bridge. This device provides connecting struts that extend perpendicular to the tension members and encircle the tension members in the manner of a collar. At these holding points, the remaining unoccupied area between the tubular sheath and the tension elements is filled by a rigid filler body in order to be able to better absorb the radially acting forces at the holding points. Vibration dampers in the vicinity of the struts prevent relatively large vibrations.
From EP 1 357 229 A1, which corresponds to U.S. Pat. No. 7,007,430, is known a tension member for cable-stayed bridges, which likewise include a number of tension elements running within a tubular sheath. It is proposed there to introduce a curable material, for example foam, into the sheath in order to avoid transverse movements of the individual tension elements within the tubular sheath. However, the uncontrollable expansion of the filler medium within the sheath tube and the adhesion of the filler medium to the tension elements turns out to be disadvantageous here, with the result that individual tension elements cannot be replaced later in the course of maintenance or repair or subsequent reinforcement. The filler material, too, cannot be removed later, or only with disproportionately great effort. Moreover, there exists the danger that the filler material will be destroyed by relative movements of the sheath and the tension elements as a result of temperature or load changes.
Another option for keeping the tubular sheath spaced apart from the individual tension elements is known from EP 0 169 276 A1. There, a tubular element extends over the entire length of the tension member parallel to its axis between the tubular sheath and the bundle of tension elements; the tubular element can be brought into contact with both the inside of the tubular sheath and the tension elements by filling with a filler material. In this way, a linear support of the tubular sheath is achieved along the entire cable stay.
The extension of the tubular element over the full length of the tension member here proves to be disadvantageous. Firstly, this requires relatively large quantities of filler material, which proves to be uneconomical. Moreover, due to the great length of the tubular element and its flow resistance, high pressures are necessary to completely fill the tube with a filler material. In order to be able to withstand these pressures, the tubular element must be reinforced in a correspondingly resource-intensive manner. But the mechanical equipment necessary for filling must also be able to generate such high pressures. Thus, considerable costs for acquisition and operation must be expected in terms of equipment.
In order to even achieve complete filling of the tubular element under reasonably moderate pressure conditions, one must resort to a low-viscosity filler material with the disadvantage that even the smallest leaks in the tubular element can result in the filling running out. In contrast, the use of granular material is ruled out because it cannot be pushed in over the full length of the tubular element.
From a statics standpoint, the tension member disclosed in EP 0 169 276 A1 is not capable of absorbing forces acting at a point on the tubular sheath, such as those from connecting struts between the individual cable stays, since firstly the filler material can yield in the axial direction under radial compressive forces, and secondly the remaining unoccupied area of the tubular sheath is not completely filled, but instead is only partially filled.